Smagorinsky Subgrid Scale Research Articles

A comparative study of three-dimensional discrete velocity set in LBM for turbulent flow over bluff body

The work presented in this paper investigates the efficiency of various (i.e., $$D_{3}Q_{15}$$ , $$D_{3}Q_{19}$$ , and $$D_{3}Q_{27}$$ ) three-dimensional discrete velocity models of lattice Boltzmann method (LBM) for the simulation of turbulent flow past over a bluff body. The numerical setup consists of a square cylinder confined in a rectangular duct. Large eddy simulation (LES) model has been used for the simulation of turbulent eddies. The small-scale turbulent structures were resolved by using the conventional Smagorinsky subgrid-scale (SGS) model. The computations have been carried out on a uniform Cartesian grid. The results for different turbulent statistics have presented and compared with the available experimental and previous numerical (based on finite volume approach) results for Reynolds number $$Re_{d} = 3000$$ . The algorithm developed for the present simulation has been parallelized to run on Graphical Processing Unit (GPU) parallel platform.

Studying the flow dynamics and heat transfer of stranded conductor cables using large eddy simulations

Stranded cables are widely used in applications where their heat transfer and fluid dynamics are important, but they have not been extensively studied. This paper investigates, using large eddy simulations with the dynamic Smagorinsky sub-grid scale model, a helically wound stranded conductor cable in comparison to a circular cylinder at a Reynolds number of 1000 and Prandtl number of 0.7. The cylinder and the cable were normal to the flow. The triply decomposed heat transport equations were derived, and proper orthogonal decomposition was applied to the fluctuating vorticity and temperature fields to determine the total, coherent, and incoherent terms in the heat transport equations. The results showed that the stranded cable, relative to circular cylinder, has (i) three-dimensional mean flow and heat transfer, especially within and around recirculation region, (ii) 9% higher drag and 8% higher base pressure magnitude, (iii) near-stagnant flow in the gaps between the strands, which results in a significant variation in the local Nusselt number, (iv) ∼15% lower span-wise averaged local Nusselt number in the attached boundary layer, suggesting that surface modifications should be addressed to enhance heat transfer, (v) ∼36° variation in the separation angle along the span, (vi) 12% higher turbulent kinetic energy and 39% higher spanwise normal Reynolds stresses, (vii) insignificant difference in shedding frequency, suggesting similar flow induced vibrations to the cylinder, (viii) asymmetry in the flow and heat fields around the x axis, (ix) significantly different coherent temperature fields and dynamics, and (x) in general, high heat energy transport close to the cable rear side.

Large Eddy Simulation of low Reynolds number turbulent hydrogen jets - Modelling considerations and comparison with detailed experiments

The main objective of this work is to apply Large Eddy Simulation (LES) on hydrogen subsonic round jets in order to evaluate modelling strategies and to provide guidelines for similar applications. The ADREA-HF code and the experiments conducted by Sandia National Laboratories are used for that purpose. These experiments are very suitable for LES studies because turbulent fluctuations have been measured which is something rare in hydrogen experiments. Hydrogen is released vertically from a small orifice of 1.91 mm diameter into stagnant environment. Three experimental cases are simulated with different Reynolds number at the release area, namely 885, 1360 and 2384. Hydrogen mass fraction and velocity mean values and fluctuations are compared against the experimental data. Several grid resolutions are used to assess the effect on the results, using mainly the Smagorinsky subgrid scale model. In the lowest Reynolds number case, an Implicit LES code, used independently from a different scientific group, is also tested. In this case, the performance of the RNG-LES subgrid scale model of the ADREA-HF code is also examined. Additionally, the effect of Smagorinsky constant and of the Van Driest correction is evaluated. The amount of the resolved turbulence and of the velocity spectra are presented. Finally, the effect of the release modelling is discussed. The analysis shows that even a coarse discretization of the release area can give acceptable results for hydrogen safety engineering applications. However, dense grids are required for the more accurate prediction of the turbulent characteristics. The two LES codes gave similar results and the overall agreement with the experiment was satisfactory.

Validation study for Large-Eddy Simulation of Forest Flow

The publication presents Large-Eddy Simulation (LES) of flow over a reduced-scale wind tunnel model of a forest canopy. The final aim of the study is to determine factors responsible for damage in forests by strong winds. The wind tunnel forest was represented by an open-porous foam material for the crown layer and wooden dowels for the trunk layer. The forest model was installed in the open test section of a Goettingen-type wind tunnel and Particle Image Velocimetry (PIV) measurements were made for the acquisition of the flow field data. The numerical simulations were performed with OpenFOAM®. The forest was modelled by an additional sink term in the momentum transport equations based on the leaf area density and a characteristic drag coefficient for the underlying tree specimen. Large-eddy simulations with different subgrid-scale (SGS) turbulence models were carried out and compared to wind tunnel data. The Smagorinsky SGS model outperformed the dynamic Lagrangian SGS model in the windward edge region (within a distance of approximately 2 tree heights from the leading edge) whereas the dynamic Lagrangian SGS model showed a better performance for regions farther downstream.

An Acoustic Source Model for Applications in Low Mach Number Turbulent Flows, Such as a Large-Scale Wind Turbine Blade

Aerodynamic noise from wind turbine blades is one of the major hindrances for the widespread use of large-scale wind turbines generating green energy. In order to more accurately guide wind turbine blade manufacturers to optimize the blade geometry for aerodynamic noise reduction, an acoustic model that not only understands the relation between the behavior of the sound source and the sound generation, but also accounts for the compressibility effect, was derived by rearranging the continuity and Navier–Stokes equations as a wave equation with a lump of source terms, including the material derivative and square of the velocity divergence. Our acoustic model was applied to low Mach number, weakly compressible turbulent flows around NACA0012 airfoil. For the computation of flow fields, a large-eddy simulation (LES) with the dynamic Smagorinsky subgrid scale (SGS) model and the cubic interpolated pseudo particle (CIP)-combined unified numerical procedure method were conducted. The reproduced turbulent flow around NACA0012 airfoil was in good agreement with the experimental data. For the estimation of acoustic fields, our acoustic model and classical sound source models, such as Lighthill and Powell, were performed using our LES database. The investigation suggested that the derived material derivative of the velocity divergence plays a dominant role as sound source. The distribution of the sources in our acoustic model was consistent with that of the classical sound source models. The sound pressure level (SPL) predicted based on the above-mentioned LES and our newly derived acoustic model was in reasonable agreement with the experimental data. The influence of the increase of Mach number on the acoustic field was investigated. Our acoustic source model was verified to be capable of treating the influence of Mach numbers on the acoustic field.

Large Eddy Simulations of Turbulent Flows at Supercritical Pressure in a Vertical Heated Pipe Using Unstructured Cartesian Grids with Local Refinement

This paper presents the results of numerical simulation of turbulent flow of CO2 at supercritical pressure (SCP) in a uniform heated vertical tube using the large eddy simulation (LES) with the Smagorinsky subgrid scale (SGS) model. Unstructured Cartesian grids with local anisotropic refinement were used for modeling hydrodynamics and heat transfer in a circular pipe. Such grids unlike structured grids in cylindrical coordinates allow one to construct more optimal meshes for the LES method. In addition, such meshes allow one to simulate flows in channels of arbitrary cross-section. To verify the proposed methodology the results of the direct numerical simulation were used. Numerical simulations have been performed using the “in-house” CFD code ANES.

Numerical and experimental investigation of the pressure fluctuation in a mixed-flow pump under low flow conditions

In this paper, the large eddy simulation is utilized to simulate the flow field in a mixed-flow pump based on the standard Smagorinsky subgrid scale model, which is combined with the experiments to investigate pressure fluctuations under low flow conditions. The experimental results indicated that the amplitude of fluctuation at the impeller inlet is the highest, and increases with the reduction of the flow rate. The main frequencies of pressure fluctuation at the impeller inlet, impeller outlet, and vane inlet are blades passing frequency, while the main frequency at the vane outlet changes with the flow rate. The results of the simulation showed that the axial plane velocity at impeller inlet undergoes little change under 0.8 Qopt. In case of 0.4 Qopt, however, the flow field at impeller inlet becomes complicated with the axial plane velocity changing significantly. The flow separation is generated at the leading edge of the suction surface at t* = 0.0416 under 0.4 Qopt, which is caused by the increase of the incidence angle and the influence of the tip leakage flow. When the impeller rotates from t* = 0.0416 to t* = 0.1249, the flow separation intensified and the swirling strength of the separation vortex is gradually increased, leading to the reduction of the static pressure, the rise of adverse pressure gradient, and the generation of backflow. The static pressure at the leading edge of the impeller recovers gradually until the backflow is reached. In addition, the flow separation is the main reason for the intensification of the pressure fluctuation.

Thermal-hydraulic study on rib and dimple structures for cooling the First Wall of DEMO

The thermal hydraulics of helium-gas cooled channels for the First Wall of DEMO structured by fully attached and semi-detached upward directed V-shaped ribs and spherical dimples in a staggered array were investigated. Large-Eddy-Simulations with the dynamic Smagorinsky sub-grid scale model were performed. The global and local heat transfer coefficient, pressure drop and cooling performance were analyzed. Compared to smooth channel flows, the global heat transfer coefficient was increased by a factor of 2.6–2.9 and the corresponding pressure drop was increased by a factor of 5.3–5.7 for the V-shaped ribs. Here, the semi-detached ribs provided the highest heat transfer and the best cooling performance. The dimpled structures increased the heat transfer by the factor of 1.1–1.4 without a significant pressure drop rise when compared to smooth channel flows, but regions of local heat transfer deterioration occur over each upstream dimple half.

Numerically investigation of ignition process in a premixed methane-air swirl configuration

Ignition process in a premixed methane-air swirl configuration is studied using a large eddy simulation method with Smagorinsky sub-grid scale model. A developed thickened flame combustion approach with two-step methane-air mechanism is used. Non-reacting mean and RMS axial, tangential and radial velocity profiles are validated against the experimental results. It is shown that the flow field consists of four zones: Inner Recirculation Zone, Inner Shear Layer, Outer Shear Layer and Corner Recirculation Zone. The mean and RMS of velocities and temperature in reacting flow are then validated against the experimental data. Large eddy simulation is used to investigate the ignition sequence by sparking in the four zones in the flow field. Flame growth, propagation and stabilization are studied for these cases. Results show that sparking in IRZ has the fastest flame growth and takes the minimum time to reach flame stabilization. Propagating flame surface in all cases has sharp flame edges, without any hysteresis for flame position. Finally, flame structures are analyzed by flame curvature and the effect of flow field velocity on the flame surface.

Effect of inclination of twin impinging turbulent jets on flow and heat transfer characteristics

This paper presents analyses of complex flows and heat transfer induced by twin oblique turbulent slot-jets of different directions (divergent, convergent or parallel) impinging a heated wall. A comparison of the heat transfer characteristics between perpendicular and three cases of twin oblique jets (parallel, convergent and divergent). The twin slot jets are located on a confining adiabatic wall at a distance of 8 slot jet width. Convective heat is investigated numerically examining the effect of Reynolds number (Re) and jet inclination angle (α). This problem is relevant to a wide range of practical applications including nuclear engineering devices, manufacturing, material processing, electronic cooling, drying paper or textile, tempering of glass, etc. All computations are performed using two dimensional large eddy simulations (LES) approach with Smagorinsky sub-grid scale (SGS) models. For all directions and inclinations of the jets, the location of the stagnation points is changed and hence, the location and magnitude of the maximum Nusselt number on the heated wall vary. When Reynolds number increases, Nusselt number is improved for all types of inclination. The averaged Nusselt number shows that the perpendicular impingement gives better heat transfer than that of the oblique jets. The poor heat transfer is obtained for the parallel oblique jets. For the same angle, divergent jets give smallest heat transfer than the convergent jets.

Influence of inter‐particle collisions and agglomeration on cyclone performance and collection efficiency

AbstractFor further improving the understanding of particle separation in gas cyclones including the effect of agglomeration, numerical calculations based on the coupled Euler/Lagrange approach were conducted using an open source CFD code. The gas phase was simulated by LES (large eddy simulations) combined with a dynamic Smagorinsky sub‐grid‐scale (SGS) model for resolving the highly anisotropic turbulence structure. Two‐way coupling (i.e., the influence of the particles on the fluid flow field) was accounted for in the momentum equations as well as in the SGS turbulence. Solid particle agglomeration was modelled on the basis of the stochastic inter‐particle collision model.1 In that respect, it is also important to consider the effect of particle dispersion by SGS turbulence and wall roughness in particle‐wall collisions,2 which eventually will also modify inter‐particle collisions and agglomeration. For describing the agglomeration phenomenon, two different approaches were tested. The first one, the so‐called sphere model, considers that the agglomerate diameter is calculated from the sum of the volume of the involved primary particles (volume equivalent diameter). In the second approach, the agglomeration history model3 allows the calculation of the agglomerate porosity, which is used to calculate a more suitable hydrodynamic diameter and, therefore, allows a better prediction of the motion of newly formed agglomerates. The effect of inter‐particle collision and agglomeration on the performance of a cyclone was analyzed in a hypothetical study considering a high efficiency Stairmand cyclone.

Large Eddy simulations of flow around tandem circular cylinders in the vicinity of a plane wall

To investigate the three-dimensional flow around free-spanning, tandem marine pipelines, Large Eddy Simulations (LES) with Smagorinsky subgrid scale model are performed using the open-source code OpenFOAM. Two circular cylinders in tandem arrangement are placed in the vicinity of a rigid, horizontal, plane wall. The cylinders are immersed in a steady current with a logarithmic boundary layer profile at an intermediate, subcritical Reynolds number (Re = 1.31 × 104). The non-dimensional distances between the cylinder centres are 2 and 5 (L/D = 2 and 5) and the gap to diameter ratios are G/D = 0.6 and 1, where gap G is the distance between the bottom of the cylinders and the wall. The present results are analysed through the values of drag and lift coefficients, as well as by the details of the flow fields in the wake of the cylinders. The results are compared with experimental results of the flow around tandem cylinders in an unlimited fluid for both L/D = 2 and 5, showing that, at chosen gaps, at L/D = 2, the flow belongs to the reattachment regime, and at L/D = 5 to the co-shedding regime. Compared with the case of a single cylinder near a plane wall, the flow around the tandem cylinders at G/D = 1 belongs to the wide gap regime, while at G/D = 0.6, tandem yields a stronger interaction with the wall than in the case of one cylinder. The presence of the wall modifies the flow in the spacing between the two cylinders, giving it some characteristics of the extended body flow regime.

Application of a Steady Meandering River with Piers Using a Lattice Boltzmann Sub-Grid Model in Curvilinear Coordinate Grid

A sub-grid multiple relaxation time (MRT) lattice Boltzmann model with curvilinear coordinates is applied to simulate an artificial meandering river. The method is based on the D2Q9 model and standard Smagorinsky sub-grid scale (SGS) model is introduced to simulate meandering flows. The interpolation supplemented lattice Boltzmann method (ISLBM) and the non-equilibrium extrapolation method are used for second-order accuracy and boundary conditions. The proposed model was validated by a meandering channel with a 180° bend and applied to a steady curved river with piers. Excellent agreement between the simulated results and previous computational and experimental data was found, showing that MRT-LBM (MRT lattice Boltzmann method) coupled with a Smagorinsky sub-grid scale (SGS) model in a curvilinear coordinates grid is capable of simulating practical meandering flows.

Large eddy simulations of the all-speed turbulent jet flow using 3-D CFD code GASFLOW-MPI

Turbulent flow at a wide range of Mach numbers may occur during design based accidents (DBA) or severe accidents in nuclear power plants (NPPs), for instinct, incompressible gas mixing flow (subsonic), gas mixture critical flow (transonic) or detonation (supersonic). All of them need to be simulated in safety analysis of NPP containments, therefore we need the computer code to have the capability to simulate all-speed flow. GASFLOW-MPI is a well-developed parallel all-speed CFD software used to predict gas turbulent mixing, chemical combustion and other related thermal-hydraulic phenomena in containments of NPPs. In order to validate the all-speed turbulent flow capability of GASFLOW-MPI, four turbulent jet cases over a broad range of Mach number have been performed in this paper. The large eddy simulation (LES) turbulent model is employed to capture more details of turbulence and flow features in this study. The standard Smagorinsky sub-grid scale model is utilized to model the unresolved turbulent behaviors at sub-grid scale. Both the instantaneous and time-averaged flow fields are analyzed as well as turbulent statistics to compare with the experimental data sets. The simulation results are in good agreement with the experimental data sets available in the literature. The shear layer instability is observed in the instantaneous flow field which leads to the end of the potential core and then the centerline velocity begins to decay. The size of the potential core and the decay rate of the centerline velocity are consistent well with the experiments. The turbulence intensity in the fully developed region agree well with the experimental data and the velocity power spectrum reveals the -5/3 law. Simulation results show that the compressibility has a significant effect on a free shear layer growing rate which is consistent with the experiments. The complex shock wave structure is also captured exactly by the second-order total variation diminishing (TVD) scheme. GASFLOW-MPI code has been successfully validated using the experimental data of turbulent jet flow at the wide regime of Mach numbers.

Çinlar subgrid scale model for large eddy simulation

We construct a new subgrid scale (SGS) stress model for representing the small scale effects in large eddy simulation (LES) of incompressible flows. We use the covariance tensor for representing the Reynolds stress and include Clark’s model for the cross stress. The Reynolds stress is obtained analytically from Çinlar random velocity field, which is based on vortex structures observed in the ocean at the subgrid scale. The validity of the model is tested with turbulent channel flow computed in OpenFOAM. It is compared with the most frequently used dynamic Smagorinsky and one-equation eddy SGS models through DNS data.

Large eddy simulation of a transitional, thermal Blasius flow at low Reynolds number

Large eddy simulation (LES) of a transitional, thermal Blasius flowfield at a sufficiently low Reynolds number along an isothermal, flat plate is presented herein. Prior experiments observed that boundary layer transition is induced by the presence of streamwise vortex instability caused by the complex interaction between thermal buoyancy and forced convection dynamics. The maximum Grashof and Reynolds numbers employed in the LES were approximately 1.05×1011 and 1.18×105, respectively. To further enhance the accuracy, computational efficiency, and numerical stability, the LES solved the low-Mach number compressible flow governing equations, which included fluctuating density effects and pressure-density decoupling. For the subgrid scale (SGS) closure, a locally dynamic Smagorinsky SGS model was implemented into the LES solver to enable the backscatter phenomenon intrinsic to transitional boundary layer flows. The LES accurately predicted the onset of streamwise vortex instability and the eventual three-dimensional vortex breakdown of the underlying mean flow into a fully developed turbulent boundary layer, when compared to previously measured data. In the developed turbulence region, quadrant analyses indicated Q2 and Q4 events dominated the contribution to the Reynolds shear stress in the near-wall region, whereas Q1 and Q3 events contributed considerably to the wall-normal turbulent heat flux. And, as a result of the instability within the conduction layer, the layer erupts and intermittently releases buoyant thermal plumes. These recurring intermittent events inside the conduction layer deflect and deform the flowfield quantities near the wall, resulting in a plurality of peculiar peaks and shear layers inside the boundary layer. Furthermore, these buoyant thermal plumes are the dominant turbulence production mechanism farther away from the wall in the downstream region of the developed turbulent boundary layer flow. Near the wall, however, forced convection turbulent flow effects were observed, in which the shear production term was the primary contributor to the generation of turbulent kinetic energy, and quasi-streamwise and horseshoe-like vortex structures were observed in the developed turbulence region.

Large Eddy Simulations of flow around two circular cylinders in tandem in the vicinity of a plane wall at small gap ratios

Large Eddy Simulations (LES) with Smagorinsky subgrid scale model have been performed for the flow past two circular cylinders in tandem placed in the vicinity of a horizontal plane wall at very small gap ratios, namely G∕D=0.1,0.3 and 0.5, in three-dimension (3D). The ratio of cylinder center-to-center distance to cylinder diameter, or the pitch ratio, L∕D, considered in the simulations is L∕D=2 and 5. This work serves as an extension of Abrahamsen Prsic et al. (2015). In essence, six sets of simulations have been performed in the subcritical Reynolds number regime at Re=1.31×104. Our major findings can be summarized as follows. (1) At both pitch ratios, the wall proximity has a decreasing effect on the mean drag coefficient of the upstream cylinder. At L∕D=2, the mean drag coefficient of the downstream cylinder is negative since it is located within the drag inversion separation distance. (2) At L∕D=2, a squarish cavity-like flow exists between the cylinders and the flow circulates within the cavity. A long lee-wake recirculation zone is found behind the downstream cylinder at G∕D=0.1. However, a much smaller lee-wake recirculation zone is noticed at L∕D=5 with G∕D=0.1. (3) At L∕D=2, the reattachment is biased to the bottom shear layer due to the deflection from the plane wall, which leads to the formation of the slanted squarish cavity-like flow. At both pitch ratios, as G∕D becomes smaller, stronger vortices are found between the two cylinders. Less intense vortices are observed in the near wake of the downstream cylinder due to the vortex shedding suppression of the neighboring wall.

On the refinement of the rotation rate based Smagorinsky model using velocity field gradients

In this paper, a gradient-based refinement of the rotation rate-based Smagorinsky (RoSM) subgrid scale (SGS) model is presented. The refined model satisfies the Galilean invariance condition generally, without any assumptions. The suggested model retains the advantages offered by the original RoSM, thus being simple and efficient. It provides a Smagorinsky model constant that is always positive, with low fluctuations in space and time, without the need for any numerical stability control algorithms. The validity of the proposed SGS model is shown through three test cases, namely, a turbulent channel flow, subcritical flow past a stationary cylinder, and a spatially developing free round jet. The refined RoSM provides comparable results with the dynamic Smagorinsky, while matching well to reference data. The refined RoSM is shown to be computationally efficient, being 20% faster than the dynamic Smagorinsky model.

Numerical Study of Ignition Process in Turbulent Shear-less Methane-air Mixing Layer

In this work, ignition process in a turbulent shear-less methane-air mixing layer is numerically investigated. A compressible large eddy simulation method with Smagorinsky sub-grid scale model is used to solve the flow field. Also, a thickened flame combustion model and DRM-19 reduced mechanism are used to compute species distribution and the heat release. Non-reacting mean and RMS axial velocity profiles and mean mixture fraction are validated against experimental data. Instantaneous mixture fraction contours show that the large bursts penetrate from the fuel stream into that of the oxidizer and vice versa and a random behaviour in the cross-stream direction. Flame kernel initiation, growth and propagation are analysed and compared with the experimental data. The ignition results show that the flame is not stable and blow-off occurs, but a more detailed investigation shows that local and short time flame stabilization exist during blow-off. During these local stabilization, heat release increased at the upstream edge of the flame. Most_upstream flame edge scalar analysis shows that the methane mass fraction has a dominant role in the local flame stabilization. OH, HO2, CH2O and heat release contours demonstration reveal that HO2 and CH2O mass fraction as well as the heat release reach a maximum on the border of the flame, but the maximum OH concentration is located in the middle of flame kernel.

Experimental and Computational Analysis of the Swirling Flow Generated by an Axial Counter-Rotating Swirler in a Rectangular Model Chamber Using Water Test Rig

This paper reports the particle image velocimetry (PIV) measurement of swirling flow in a confined rectangular-shaped model combustion chamber. Water is used as the working fluid, and the average profiles of axial, radial, and magnitudes of velocity are given. Flow behavior is investigated and a rebound angle term is defined to investigate the direct effects of the noncircular chamber shape. Flow behavior near the walls is discussed in detail, as are other important swirling flow features such as the appearance of corner and central toroidal recirculation zones. Additionally, experimental data are compared with simulation results. Analyses were performed via commercial software STAR-CCM+ version 9.0. The large eddy simulation (LES) dynamic Smagorinsky subgrid scale, realizable k–ε model, and k–ω shear-stress transport (SST) detached eddy version were used as simulation tools. Three different test filters of 1.0, 2.2, and 3.0 were applied to the LES to identify improvements in accuracy. The overall best turbulence model is compared to the experimental result and reliability of such model is evaluated. The ability of such model was profound within the upstream and to some extent unreliable in downstream.